(a) Field of the Invention
The present invention relates to a method for preparing an acrylic rubber latex having a bimodal particle distribution, and more particularly, to a method for preparing an acrylic impact modifier having two kinds of particle distribution by means of multi-step emulsion polymerization.
(b) Description of the Related Art
Impact modifiers used for improving the impact resistance of vinyl chloride resin include, for example, methyl methacrylate-butadiene-styrene (MBS) based resin, chloropolyethylene (CPE) based resin, and acrylic based resin. Among them acrylic based impact modifiers have good weatherability and are widely used in products that are exposed to sunlight. Particularly, for the applications in window frames where a high impact resistance and weatherability are required, a so-called acrylic graft polymer of a core-shell type is widely used in order to achieve weatherability, wherein the polymer is formed by grafting an acrylic polymer, which has excellent compatibility with vinyl chloride resin, to a rubbery core that is made by crosslinking an alkyl acrylate monomer with a crosslinking agent.
The mechanical properties of an acrylic impact modifier having a core-shell structure is greatly affected by the method of connecting rubbery core and shell through chemical bonding. One of the important factors on which impact resistance and processability of resins depend is the degree of bonding between the dispersed rubber particles and their matrix. The diameter of rubber particles dispersed into the matrix as well as the distance between the rubber particles acts as important factors which affect the processability and impact resistance.
Examples of conventional production methods of impact modifier for vinyl chloride resins include:                a multi-step emulsion polymerization process comprising a step of preparing seed rubber latex having a small particle size, followed by preparing a multilayered rubbery polymer by adding a rubbery polymer-forming monomer and a hard shell polymer-forming monomer sequentially and dividedly in multiple-steps; and        a microagglomeration process comprising steps of preparing a primary core-shell rubber latex having a small size (smaller than 1000 Å), making it grow to a desired size through an agglomeration process, and finally forming an encapsulated shell, and thereby impact modifiers having core-shell structures are obtained.        
Such latexes prepared by the above methods are acrylic-based impact modifiers having a single particle size and a single type of particle distribution, and are disclosed in U.S. Pat. Nos. 5,612,413 and 5,312,575. When dispersed into a matrix polymer, polymers prepared by the multi-step emulsion polymerization process are dispersed with the state of particle size of final latices, whereas impact modifiers prepared by the micro-agglomeration process are re-dispersed with the state of primary particles, that is, a state of initial particles.
Generally, impact resistance of vinyl chloride resins is known to be satisfactory when the particle size of impact modifier dispersed into a matrix is about 2000 Å and the distances between impact modifier particles are maintained to be under a specific value.